Unable to send sms using arduino nano and sim900a A6 GSM module

I am using an Arduino nano 328p and sim900A A6 GPRS/GSM module.I want to send an sms from the module to my phone.I have connected the tx of arduino to u_rxd of gsm module , rx of arduino to u_txd of module and gnd of arduino to gnd of module.I am powering both the arduino and gsm modules using usb cables.The code I am using is this:

char phone_no[]="+91xxxxxxxxxx";

void setup() {
Serial.begin(9600);
delay(300); 

Serial.println("AT+CMGF=1");    
delay(2000);
Serial.print("AT+CMGS=\"");
Serial.print(phone_no); 
Serial.write(0x22);
Serial.write(0x0D);  // hex equivalent of Carraige return    
Serial.write(0x0A);  // hex equivalent of newline
delay(2000);
Serial.print("GSM A6 test message!");
delay(500);
Serial.println (char(26));//the ASCII code of the ctrl+z is 26

}

void loop()
{

}

The serial monitor shows the AT commands being typed but neither is there any response to the commands nor is sms received by my phone.When I try to upload a blank sketch to the arduino and send AT commands through the serial monitor,the commands simply vanish after I press enter.Please help!!!!

sim900A A6 GPRS/GSM

The SIM900A and the A6 are made by different companies. You need to be clear on which device you have. Link?

We also need a drawing of how you have it connected up. Do you have the device attached do D0 & D1, AND you are using serial monitor?

What I always suggest in cases like this is upload a simple serial relay sketch to the Arduino, then send the AT commands manually and see what response you are getting.